In the last chapter we developed a model using modes of transportation
to illustrate the concept of inheritance. In this chapter we will use that
model to illustrate some of the finer points of inheritance and what it
can be used for. If it has been a while since you read and studied chapter
7, it would be good for you to return to that material and review it in
preparation for a more detailed study of the topic of inheritance.

REORGANIZED FILE STRUCTURE

Example program ------> INHERIT1.CPP

A close examination of the file named INHERIT1.CPP will reveal that
it is identical to the program developed in chapter 7 named ALLVEHIC.CPP
except that the program text is rearranged. The biggest difference is that
some of the simpler methods in the classes have been changed to inline
code to shorten the file considerably. In a practical programming situation,
methods that are this short should be programmed inline since the actual
code to return a simple value is shorter than the code required to send
a message to a non-inline method.

The only other change is the reordering of the classes and associated
methods with the classes all defined first, followed by the main program.
This puts all class interface definitions on a single page to make the
code easier to study. The implementations for the methods are deferred
until the end of the file where they are available for quick reference
but are not cluttering up the class definitions which we wish to study
carefully in this chapter. This should be an indication to you that there
is considerable flexibility in the way the classes and methods can be arranged
in C++. Of course you realize that this violates the spirit of C++ and
its use of separate compilation, but is only done here for convenience.
The best way to package all of the example programs in this chapter is
like the packaging illustrated in chapter 7.

As mentioned before, the two derived classes, car and truck,
each have a variable named passenger_load which is perfectly legal.
The car class has a method of the same name, initialize(),
as one declared in the super-class named vehicle. The rearrangement
of the files in no way voids this allowable repeating of names.

After you have convinced yourself that this program is truly identical
to the program named ALLVEHIC.CPP from chapter 7, compile and execute it
with your compiler to assure yourself that this arrangement is legal. Due
to this means of code packaging, you will not need a "make" file
or a "project" capability to compile and execute this code. This
code arrangement is designed to make it easy to compile and execute the
example programs in this chapter.

THE SCOPE OPERATOR

Because the method initialize() is declared in the derived car
class, it hides the method of the same name which is part of the base
class, and there may be times you wish to send a message to the method
in the base class for use in the derived class object. This can be done
by using the scope operator in the following manner in the main program;

sedan.vehicle::initialize(4, 3500.0);

As you might guess, the number and types of parameters must agree with
those of the method in the base class because it will respond to the message.

WHAT IS PROTECTED DATA?

If the data within a base class were totally available in all classes
inheriting that base class, it would be a simple matter for a programmer
to inherit the base class into a derived class and have free access to
all data in the parent class. This would completely override the protection
afforded by the use of information hiding. For this reason, the data in
a class are not automatically available to the methods of an inheriting
class. There are times when you may wish to automatically inherit all variables
directly into the subclasses and have them act just as though they were
declared as a part of those classes also. For this reason, the designer
of C++ has provided the keyword protected.

In the present example program, the keyword protected is given
in line 6 so that all of the data of the vehicle class can be directly
imported into any derived classes but are not available outside of the
base class or derived classes. As we have mentioned before, all data are
automatically defaulted to private at the beginning of a class if
no specifier is given.

You will notice that the variables named wheels and weight
are available to use in the method named initialize() in lines
83 through 88 just as if they were declared as a part of the car class
itself. They are available because they were declared protected in
the base class. Of course, they would be available here if they had been
decared public in the base class, but then they would be available
outside of both classes and we would lose our protection. Note that the
two variables are available for use in the base class as illustrated in
lines 77 and 78. We can now state the rules for the three means of defining
variables and methods.

private - The variables and methods are not available to any
outside calling routines, and they are not available to any derived classes
inheriting this class.

protected - The variables and methods are not available to any
outside calling routines, but they are directly available to any derived
class inheriting this class.

public - All variables and methods are freely available to all
outside calling routines and to all derived classes.

You will note that these three means of definition can also be used
in a struct type. The only difference with a struct is that
everything defaults to public until one of the other keywords is
used.

Be sure to compile and execute this program before continuing on to
the next example program.

WHAT IS PRIVATE DATA?

Example program ------> INHERIT2.CPP

Examine the file named INHERIT2.CPP where the data in the base class
is permitted to use the private default because line 6 is commented
out. In this program, the data is not available for use in the derived
classes, so the only way the data in the base class can be used is by sending
messages to methods in the base class, even within the derived class.

It seems a little silly to have to call methods in the base class to
access the data which is actually a part of the derived class, but that
is the way C++ is defined to work. This would indicate to you that you
should spend some time thinking about how any class you define will be
used. If you think somebody may wish to inherit your class into a new class
and expand it, you should make the data members protected so they
can be easily used in the new class. Lines 86 and 87 are invalid now since
the members are not visible, but line 88 now does the job they did before
they were hidden by calling the public method of the base class. Line 104
is also changed because of the hidden data. You will notice that the data
is still available in lines 77 and 78 just as they were before because
the member variables are protected in the vehicle class. Be sure
to compile and execute this program.

HIDDEN METHODS

Example program ------> INHERIT3.CPP

Examine the file named INHERIT3.CPP carefully and you will see that
it is a repeat of the first example program in this chapter with a few
minor changes.

You will notice that the derived classes named car and truck
do not have the keyword public prior to the name of the base
class in the first line of each class declaration. The keyword public,
when included prior to the base class name, makes all of the methods defined
in the base class available for use in the derived class at the same security
level as they were defined at in the base class. Therefore, in the previous
program, we were permitted to call the methods defined as part of the base
class from the main program even though we were working with an object
of one of the derived classes.

In this program, all entities are inherited as private due to
the use of the keyword private prior to the name of the base class.
They are therefore unavailable to any code outside of the derived class.
The general rule is that all elements are inherited into the derived class
at the most restrictive of the two restrictions placed on them, one being
the definition in the base class and the other being the restriction on
inheritance. This defines the way the elements are viewed outside of the
derived class.

The elements are all inherited into the derived class such that they
have the same level of protection they had in the base class, as far as
their visibility restrictions within the derived class. We have returned
to the use of protected data instead of private in the base
class, therefore the member variables are available for use within the
derived class.

In the present program, the only methods available for objects of the
car class, are those that are defined as part of the class itself,
and therefore we only have the methods named initialize() and passengers()
available for use with objects of class car.

When we declare an object of type car, according to the definition
of the C++ language, it contains three variables. It contains the one defined
as part of its class named passenger_load and the two that are part
of its parent class, wheels and weight. All are available
for direct use within its methods because of the use of the keyword protected
in the base class. The variables are a part of an object of class car
when it is declared and are stored as part of the object.

The observant student will notice that several of the output statements
have been commented out of the main program since they are no longer legal
or meaningful operations. Lines 57 through 59 have been commented out because
the methods named get_weight() and wheel_loading() are not
available as members of the car class because we are using private
inheritance. You will notice that initialize() is still available
but this is the one in the car class, not the method of the same
name in the vehicle class.

USING THE TRUCK CLASS

Moving on to the use of the truck class in the main program,
we find that lines 63 and 65 are commented out for the same reason as given
above, but lines 66 and 67 are commented out for an entirely different
reason. Even though the method named efficiency() is available and
can be called as a part of the truck class, it cannot be used because
we have no way to initialize the wheels or weight of the
truck object. We can get the weight of the truck object,
as we have done in line 102, but since the weight has no way to
be initialized, the result is meaningless and lines 66 and 67 are commented
out.

Private inheritance is very similar to using an embedded object
and, in fact, is very rarely used. Until you gain a lot of experience with
C++ and the proper use of Object Oriented Programming, you should use public
inheritance exclusively. There is probably no reason to ever use private
or protected inheritance. They were probably added to the language
for completeness.

Be sure to compile and execute this example program to see that your
compiler gives the same result. It would be a good exercise for you to
reintroduce some of the commented out lines to see what sort of an error
message your compiler issues for these errors.

INITIALIZING ALL DATA

Example program ------> INHERIT4.CPP

If you will examine the example program named INHERIT4.CPP, you will
find that we have fixed the initialization problem that we left dangling
in the last example program. We also added default constructors to each
of the classes so we can study how they are used when we use inheritance
and we have returned to the use of public inheritance.

When we create an object of the base class vehicle, there is
no problem since inheritance is not a factor. The constructor for the base
class operates in exactly the same manner that all constructors have in
previous chapters. You will notice that we create the unicycle object
in line 47 using the default constructor and the object is initialized
to the values contained in the constructor. Line 49 is commented out because
we no longer need the initialization code for the object.

When we define an object of one of the derived classes in line 57, the
sedan, it is a little different because not only do we need to call
a constructor for the derived class, we have to worry about how we get
the base class initialized through its constructor also. Actually, it is
no problem because the compiler will automatically call the default constructor
for the base class unless the derived class explicitly calls another constructor
for the base class. We will explicitly call another constructor in the
next example program, but for now we will only be concerned about the default
constructor for the base class that is called automatically.

ORDER OF CONSTRUCTION

The next problem we need to be concerned about is the order of construction,
and it is easy to remember if you remember the following statement, "C++
classes honor their parents by calling their parents constructor before
they call their own." The base class constructor will be called before
the derived class constructor. This makes sense because it guarantees that
the base class is properly constructed when the constructor for the derived
class is executed. This allows you to use some of the data from the base
class during construction of the derived class. That was a mouthful, but
if you spend a little time with this concept, it will make a lot of sense,
and you will not easily forget it. In this case, the vehicle part
of the sedan object is constructed, then the local portions of the
sedan object will be constructed, so that all member variables are
properly initialized. This is why we can comment out the initialize
method in line 59. It is not needed.

When we define a semi object in line 66, it will also be constructed
in the same manner. The constructor for the base class is executed, then
the constructor for the derived class will be executed. The object is now
fully defined and useable with default data in each member. Lines 68 and
69 are therefore not needed.

The remainder of this program should be no problem for you to understand
except for the order of destruction of the various objects.

HOW ARE THE DESTRUCTORS EXECUTED?

As the objects go out of scope, they must have their destructors executed
also, and since we didn't define any, the default destructors will be executed.
Once again, the destruction of the base class object named unicycle
is no problem, it's destructor is executed and the object is gone.
The sedan object however, must have two destructors executed to
destroy each of its parts, the base class part and the derived class part.
It should not be too much of a surprise that the destructors for this object
are executed in reverse order from the order in which they were constructed.
In other words, the object is dismantled in the opposite order from the
order in which it was assembled. The derived class destructor is executed
first, then the base class destructor is executed and the object is removed
from they system.

Remember that every time an object is defined or created, every portion
of it must have a constructor executed on it. Every object must also have
a destructor executed on each of its parts when it is destroyed in order
to properly dismantle the object.

Be sure to compile and execute this program following your detailed
study of it.

INHERITANCE WHEN CONSTRUCTORS ARE USED

Example program ------> INHERIT5.CPP

Examine the example program named INHERIT5.CPP for yet another variation
to our basic program, this time using constructors that are more than just
the default constructors. You will notice that each class has another constructor
declared within it. The additional constructor added to the vehicle
class in lines 12 through 14 is nothing special, it is just like some
of the constructors we have studied earlier in this tutorial. It is used
in line 59 of the main program where we define unicycle with two
values passed in to be used when executing this constructor.

The constructor for the car class which is declared in lines
28 through 31 is a bit different, because we pass in three values. One
of the values, the one named people, is used within the derived
class itself to initialize the member variable named passenger_load.
The other two literal values however, must be passed to the base class
somehow in order to initialize the number of wheels and the weight.
This is done by using a member initializer, and is illustrated in this
constructor. The colon near the end of line 28 indicates that a member
initializer list follows, and all entities between the colon and the opening
brace of the constructor body are member initializers. The first member
initializer is given in line 29 and looks like a constructor call to the
vehicle class that requires two input parameters. That is exactly
what it is, and it calls the constructor for the vehicle class and
initializes that part of the sedan object that is inherited from
the vehicle class. We can therefore control which base class initializer
gets called when we construct an object of the derived class.

The next member initializer, in line 30, acts kind of like a constructor
for a simple variable. By mentioning the name of the variable and including
a value of the correct type within the parentheses, that value is assigned
to that variable even though the variable is not a class, but a simple
predefined type. This technique can be used to initialize all members of
the derived class or any portion of them. When all of the members of the
member initializer list are executed, the code within the braces is executed.
In this case, there is no code within the executable block of the constructor.
The code within the braces would be written in a normal manner for a constructor.

WHAT ABOUT THE ORDER OF EXECUTION?

This may seem to be very strange, but the elements of the member initializer
list are not executed in the order in which they appear in the list. The
constructors for the inherited classes are executed first, in the order
of their declaration in the class header. When using multiple inheritance,
several classes can be listed in the header line, but in this program,
only one is used. The member variables are then initialized, but not in
the order as given in the list, but in the order in which they are declared
in the class. Finally, the code within the constructor block is executed,
if there is any code in the block.

There is a good reason for this seemingly strange order. The destructors
must be executed in reverse order from the construction order, but if there
are two constructors with different construction order defined, which should
define the destruction order? The correct answer is neither. The system
uses the declaration order for construction order and reverses it for the
destruction order.

You will notice that the truck class uses one initializer for
the base class constructor and two member initializers, one to initialize
the passenger_load, and one to initialize the payload. The
body of the constuctor, much like the car class, is empty.

The two constructors in the car class and the truck class
are called to construct objects in lines 69 and 78 for a car object
and a truck object as illustrations in this example program.

The remainder of this program should be easy for you to folow. Be sure
to compile and execute this program before moving on.

POINTERS TO AN OBJECT AND AN ARRAY OF OBJECTS

Example program ------> INHERIT6.CPP

Examine the example program named INHERIT6.CPP for examples of the use
of an array of objects and a pointer to an object. In this program, the
objects are instantiated from an inherited class and the intent of this
program is to illustrate that there is nothing magic about a derived class.
A class acts the same whether it is a base class or a derived class.

This program is identical to the first program in this chapter until
we get to the main() program where we find an array of 3 objects
of class car declared in line 53. It should be obvious that any
operation that is legal for a simple object is legal for an object that
is part of an array, but we must be sure to tell the system which object
of the array we are interested in by adding the array subscript as we do
in lines 58 through 64. The operation of this portion of the program should
be very easy for you to follow, so we will go on to the next construct
of interest.

You will notice, in line 68, that we do not declare an object of type
truck but a pointer to an object of type truck. In order
to use the pointer, we must give it something to point at which we do in
line 70 by dynamically allocating an object. Once the pointer has an object
to point to, we can use the object in the same way we would use any object,
but we must use the pointer notation to access any of the methods of the
object. This is illustrated for you in lines 76 through 80, and will be
further illustrated in the example program of chapter 12 in this tutorial.

Finally, we deallocate the object in line 81. You should spend enough
time with this program to thoroughly understand the new material presented
here, then compile and execute it.

THE NEW TIME CLASS

We began a series of nontrivial classes in chapter 5 where we developed
a date class, then a time class, and finally a newdate
class in the last chapter. Now it is your turn to add to this series.
Your assignment is to develop the newtime class which inherits the
time class and adds a new member variable named seconds_today
and a method to calculate the value of seconds since midnight to fill the
variable.

PROGRAMMING EXERCISES

Remove the comment delimiters from lines 57 through 59 of INHERIT3.CPP
to see what kind of errors are reported.

Add cout statements to each of the constructors of INHERIT4.CPP
to output messages to the monitor so you can see the order of sending messages
to the constructors.